N412PD

HISTORY OF FLIGHT

On September 22, 2010, at 1552 eastern daylight time a Bell Helicopter 412EP, N412PD, operated by the New York City Police Aviation Unit, was substantially damaged during a forced landing to Jamaica Bay, near Brooklyn, New York. The certificated commercial pilot, the certificated airline transport-rated co-pilot, and 4 additional crewmembers sustained minor injuries. Visual meteorological conditions prevailed and no flight plan was filed for the flight. The public use aerial observation flight originated at New York City Police Department (NYPD) Air Operations Heliport - Floyd Bennett Field (NY22), Brooklyn, New York, at 1510.

In separate written statements, three members of the flight crew recounted the events that transpired prior to and during the accident flight. The flight departed NY22 about 1510 for a local patrol flight. About 30 minutes later, the flight was directed to return to NY22 for refueling. While on final approach to, and about 1/2-mile from the heliport at an altitude of 300 feet agl, the crew heard a loud, mechanical "bang" sound from the engine compartment of the helicopter. None of the crew heard or saw any cautions or warnings, but noted that the helicopter immediately began descending, and the co-pilot noted a loss of rotor rpm. The co-pilot transmitted a distress call to the operator's base, and the crew confirmed that the emergency flotation system was armed. About 8 to 10 seconds after the event began, the helicopter impacted the water, the floats deployed, and the windscreens shattered after a portion of the main rotor blade struck the top of the cockpit cabin. The crew subsequently shut down the engines, secured the helicopter, and were retrieved from the scene via boat.

AIRCRAFT INFORMATION

The accident helicopter was equipped with one Pratt & Whitney Canada PT6T-3D turbo-shaft engine assembly. The engine assembly was comprised of two separate power sections, which were coupled to a single reduction gearbox (RGB). Each PT6T-3D power section featured a three-stage axial and single-stage centrifugal compressor driven by a single stage compressor turbine, a reverse-flow annular combustor, and a free turning power turbine coupled to the RGB. The RGB drove the helicopter's main rotor transmission and various engine sensing and indicating components. The PT6T-3D assembly had a takeoff and continuous power limit of 1,800 shaft horsepower (SHP) and was flat-rated to 22 degrees C.

The purpose of the RGB was to reduce the power turbines' output speed to a speed suitable for helicopter operation. The RGB had a reduction ratio of 5.0 to 1, resulting in a nominal output shaft speed of 6,600 revolutions per minute (rpm), and rotated the output drive gear clockwise. The rotational speed was further reduced, at a ratio of 20.4 to 1, through an airframe transmission to a nominal speed of 324 rpm. The RGB assembly outer structure was comprised of five cast magnesium housings: output housing (most forward), diaphragm, input housing (aft), and 2 covers mounted on the input housing (left and right).

The internal components of the RGB featured a pair of main input drive shafts (left/right) that received power from each of the power sections, idler drive spur gears, clutch drive gear assemblies, and the intermediate drive helical gears (also referred to as the 2nd stage helical gears) that drove a single output drive gear (ODG) from its circumference. The output from the ODG drove the helicopter's transmission.

The accident helicopter was manufactured by Bell Helicopter in January 2009, issued a standard United States Airworthiness Certificate on March 23, 2009, and delivered to the NYPD on December 21, 2009. When the NYPD took procession of the helicopter, both the helicopter and engines had accumulated 30.3 hours total time since new (TSN) and the engines had accumulated 60 cycles since new (CSN).

According to the helicopter's discrepancy log, low oil quantity within the RGB was reported on January 9, 2010 and on June 30, 2010. The helicopter and engines had both accumulated 39.2 hours TSN and 226.7 hours TSN, respectively, on those dates. The helicopter's most recent routine maintenance inspection was completed on August 16, 2010, at 303 hours TSN, and consisted of the 25, 100, and 300-hour inspection requirements. The only engine maintenance actions during the inspection were standard oil replenishment and filter replacement. The helicopter had accumulated approximately 359 hours TSN at the time of the accident.

WRECKAGE AND IMPACT INFORMATION

The helicopter was examined by a Federal Aviation Administration inspector after it was recovered from Jamaica Bay. The helicopter had sustained impact-related damage to all four main rotor blades, underside of the fuselage, tail boom, and tail rotor blades. Additionally, a 3- by 9-inch hole centered at the top of the RGB was observed. The engine assembly, including RGB, was subsequently removed from the helicopter and forwarded to the manufacturer's facility for further examination.

Engine Assembly and RGB External Examination

Visual examination of the engine assembly revealed that neither power section exhibited any signs of fire, breaches, or impact damage. The accessory gearboxes of both power sections were attached and their respective accessories were in place and undamaged. The inlet screens were removed from each power section and reaching through the inlet case, the compressor on each power section rotated freely by hand in both directions. Tactile examination of the leading edges of the first stage compressor blades revealed no nicks, tears or damage consistent with foreign object ingestion.

The RGB exhibited an approximate 3-inch axial (longitudinal) by 9-inch lateral exit hole in the output housing in-line with the axial position of the ODG. Pieces of the output housing in the areas of hole were either missing or were fractured and pushed outwards. Looking into the RGB, the ODG was fractured with about half the helical gear outer rim portion no longer attached to the remainder of the gear assembly.

RGB Disassembly

The power sections were removed from the RGB exposing the input cavities of the output housing. Pieces of output housing debris fell from the No. 1 power section input cavity and were collected. The output housing exhibited an approximate 2-inch axial by 6.5-inch circumferential internal hole in the wall that divided the output housing center section (output cavity) with the No. 1 power section input cavity. A section representing about 1/2 of the circumference of the ODG outer rim was found sticking through the hole in the wall that divided the output housing center section with the No. 1 power section input cavity. All of the gear teeth were present and there were three distinct areas where the helical teeth were pushed and smeared in the direction of rotation. This section of the ODG had three stubs of the web arm still attached; each exhibited varying degrees of impact damage.

The No. 1 power section main input drive shaft support structure exhibited four distinctive rub marks, located radially outward from the center cavity exit hole and in the vicinity of the No. 7 flanged roller bearing, consistent with contact with the fractured piece of the ODG. The oil transfer hole on the No 1. power section input cavity was occluded with metallic debris. The No 1. power section main input drive shaft appeared undamaged and the spiral retaining ring was still engaged. The No. 2 power section input cavity was clean and undamaged, and the oil transfer tube was unobstructed. The No. 2 power section main input drive shaft appeared undamaged and the spiral retaining ring was slightly dislodged.

The output housing was examined with the remaining portion of the ODG and both intermediate drive helical gears still installed. A large amount of metallic debris was found within the center section of the output housing, including an intact bolt that was undamaged, a stud securing lobe from the housing itself, and a piece of gear tooth. The locking feature of the portion of the ODG that remained installed was intact and secure. Approximately half of the ODG (input side of the gear) was fractured through the web and missing. On the remaining portion of the ODG, all the helical gear teeth were broken and worn away near or flush to the base of the teeth. The output splines were all present, intact and appeared to be undamaged.

The installed and other recovered portions of the ODG were sent to the engine manufacturer's Material Investigation Laboratory for further analysis.

ODG Examination

The portion of the ODG that remained installed in the RGB, consisting of the majority of the helical teeth, the shaft portion, and the output splines was arbitrarily numbered 1, and the section of the ODG outer ring recovered from the RGB output housing center section was numbered 2. The helical gear portion of piece 1 was fractured at two tooth locations with the No. 1 tooth location arbitrarily chosen at one of the two fracture locations. Piece 1 consisted of teeth number from No. 1 to No. 57 in the direction rotation of the gear while piece 2 consisted of teeth number from No. 57 to No. 115 also in the direction of rotation.

Features consistent with fatigue were found on ODG piece 1 at the No. 1 helical tooth location. It was determined that the crack initiated from the root, immediately next to the chamfer of tooth No. 1, approximately 0.020-inches from the rear end face. The No. 1 tooth fracture surface also exhibited regions with a reddish-brown color in the tooth area as well as a dark-bluish color at the web. The fractures through the No. 57 tooth and the three web arms all exhibited features consistent with tensile overload. The web arms also exhibited a noticeable reduction in cross section that was consistent with tensile overload. According to the engine manufacturer, the dark-bluish color observed at the web arm locations w...